This invention relates to an improved method of making silver-surfaced metal particles, to improved particles made by such processes, and to "conductive plastic" formulations (as broadly construed, e.g. including plastics, rubbers, and resins) or electro magnetic interference and radio-frequency shielding applications, microwave gaskets, conductive adhesives other such applications.
Silver-surfaced powder has long been used as a conductive filler in "conductive plastic" formulations. For example, Ehrreich et al disclose in U.S. Pat. No. 3,202,488 a procedure for plating silver onto copper to provide such powders. It has also been known to coat aluminum with silver to form conductive particles. One problem with these powders, when incorporated into organic binders, was that they tended to became excessively electroresistive as they aged especially at elevated temperatures. Consequently, they proved to be unsuited for a great many purposes. Moreover, it was preferable in many applications that there would not be a large increase in resistance during the life cycles of the filled product.
In powders, as were made by the process of U.S. Pat. No. 3,202,488, could not be utilized suitably in many of the applications described in U.S. Pat. Nos. 3,140,342; 3,583,930; 3,609,104 and 3,194,860. In general, they did not exhibit sufficient stability at elevated temperatures or over long periods of time.
Aging and stability problems of the prior art were particularly apparent in resilient or softer systems where the conductive powders were not compressed during cure and locked into place by a rigid matrix system.
An interesting aspect of earlier work on silver-coated copper powder was that such powders were sometimes tested for stability by heating them to relatively high temperatures for short periods of time. The heat-treated material was then measured for bulk electroconductivity using two probes across a mass of the powder and this measurement was for a use in deciding whether the powder was "good". This test was considered a destructive test, in the sense that it was thought to accelerate the loss of desirable properties by the powder, and the powder was discarded after the test. The test is described in U.S. Pat. No. 3,202,488. Subsequently, such a heating procedure, when carried out on the silver-coated powder for as long as four hours at about 190.degree. C., was found to lend some additional electronconductivity stability to compositions prepared using such heat tested powder. Nevertheless a need remained for a more stable silver-coated particle with a non-noble metal core.